This invention relates to a process for the low-temperature fractionation of air wherein feed air is compressed, purified, cooled, and, divided into several component streams, is introduced into the high-pressure stage and into the low-pressure stage of a two-stage rectifying device, a first component stream being fed to the high-pressure stage and a second component stream being fed to the low-pressure stage.
Such a process is described in EP-A 0,342,436 wherein the feed air is initially compressed only to the pressure of the low-pressure stage and is divided thereafter into first and second component streams. Only the first component stream, introduced in part into the high-pressure stage, is further compressed. Although this process provides a very economical utilization of the compression energy, it is necessary to perform the removal of carbon dioxide, hydrocarbons and water from the second component stream in a separate purification stage, usually a molecular sieve station. On account of the low pressure, this molecular sieve requires large quantities of regenerating gas. In turn, such quantities then are no longer available for other purposes, particularly for an economical evaporative cooling of the cooling water needed for the precooling of the air.